While non-uniform border widths were parsed correctly, an arbitrary
side's width was picked when painting, so that each border ended up
with the same width and the widths specified in CSS were ignored.
At least for sides between non-rounded corners, using a different
border width can be reasonable, for instance at screen edges.
Different border widths around rounded corners are kind of crack,
but then it would be lame not to support it ...
https://bugzilla.gnome.org/show_bug.cgi?id=607500
